One type of conventional optical target acquisition device uses a gimbaled rotating optical system with a reflective telescope whose optical axis is parallel to, but offset from, the axis of rotation of the optical system. The target image is projected onto a single detector through a spoke-like reticle coaxial with the axis of rotation. The reticle is positioned on the gimbal and moves with it, but the detector is off the gimbal and stationary. If the target is on the axis of rotation, the detector sees light pulses of a constant frequency. If the target drifts off-axis, a frequency modulation becomes superimposed upon the pulses. This frequency modulation is demodulated and used to move the gimbals so as to realign the axis of rotation with the target. Similar mechanisms also exist that use an amplitude modulation reticle.
Another conventional type of target acquisition device uses a gimbaled but non-rotating optical system and an array of photodetectors (e.g. 128.times.128) in which the location of the target image with respect to the center of the optical axis can be electronically determined.
The rotating type of device is electronically much simpler than the array type but it has a restricted field of view (when the target image is outside the reticle, there is no target information at all), and is highly noisy because stray light can enter the detector system from an almost 180.degree. field.
Prior art in this technology includes U.S. Pat. No. 3,353,022 tro Schwartz which describes a non-tracking IF search system with a reticle separate from the detector; U.S. Pat. No. 3,872,308 to Hopson et al. which relates to a light pipe approach to relaying light energy from a reticle to a separate detector; U.S. Pat. No. 3,944,167 to Figler et al. which shows a multidetector reticle arrangement to sort out a target from decoys; and U.S. Pat. No. 4,561,611 to Sinclair et al. which concerns a fixed target seeker for a spinning projectile in which the reticle and detector are also separate.